It was both an anomaly (Michael, desperate to get Fredo out of Havana so he can find out more of what he knows, makes no effort to snatch him in America); and illogical as I said at the end of my post: Fredo, apparently deathly afraid of his brother, shows up on Michael's command, as if he were on 24-hour call).
I don't think that the "advance notice" warning meant that Fredo had been living at the compound at the time of the boathouse confrontation, and chased out only after that confrontation. I think Fredo had been living elsewhere out of fear until that point, which is why I don't understand why he suddenly showed up when summoned.